The present invention relates to an optical receiver used in an optical transmission system based on a DPSK (Differential-Phase-Shift-Keying) modulation scheme.
A modulation scheme with high reception sensitivity is effective in obtaining good signal quality in an optical fiber transmission system. DPSK modulation is one of modulation schemes exhibiting high reception sensitivity because of their unique demodulation schemes. This modulation scheme can achieve an improvement in reception sensitivity by about 3 dB as compared with a general On-Off-keying scheme such as an NRZ (Non Return to Zero) or RZ (Return to Zero) scheme. DPSK modulation is disclosed in, for example, Japanese Patent Laid-Open No. 2006-217605.
A DPSK demodulation circuit in a DPSK receiver has a problem that since it sensitively reacts to the center frequency of an input DPSK optical signal, the reception characteristic greatly deteriorates due to a slight wavelength shift.